Preheat the oven to 350° F. Line a (9×9 inch) square cake pan with parchment paper.In a bowl, whisk together the flour, baking powder, cinnamon, salt, and espresso/coffee powder. Add the sour cream (or yogurt), eggs, maple syrup, oil, and vanilla. Mix until fully combined. Fold in the chocolate pieces. To make the streusel. Combine all ingredients in a bowl and mix until a crumble forms.Pour half the batter into the prepared pan. Sprinkle the streusel over the batter. Evenly spoon the remaining batter overtop. It's OK if it's not all covered. Bake for 45-55 minutes or until the center is just set. Remove and let cool for 30 minutes before icing.Meanwhile, make the icing (the icing can be optional). In a bowl, whisk together the powdered sugar, 1-4 tablespoons of water or brewed espresso/coffee, and vanilla. Pour the icing over the cake and top with extra chocolate chunks (if desired). Let set for one hour, slice, and enjoy.